Servo valves which control the feed direction, flow rate, and pressure of a flowing fluid (e.g., gas such as air, liquid, or the like) are used. Such servo valves each have multiple ports including a feed port, an exhaust port, and the like. For example, in the case where pressure control is performed by using a servo valve, the servo valve further has a connection port connected to a controlled object the pressure of which is controlled, as well as the feed port and the exhaust port. Such a servo valve is called a three-port valve. In the case where the flowing fluid is gas, the feed port is an air inlet, and the exhaust port is an air outlet.
A known example of the three-port valve is the servo valve disclosed in Patent Literature 1. The disclosed servo valve has three ports (a feed port, a load port, and an exhaust port). In addition, the disclosed servo valve has a spool and a sleeve. The spool has first and second valve elements. The sleeve contains the spool in such a manner that the spool can move in the axial direction, and the sleeve has a feed port which feeds air to a space between the first and second valve elements. The spool further has a third valve element which are different from the first and second valve elements.